16. Basics of Laser Therapy
Darrell Wayne Freeman, Ibrahim Khansa, Molly Burns Austin, Alton Jay Burns
PHYSICS OF LASERS1
COMPONENTS OF LASERS
■ Excitation mechanism
■ Medium
• Can be solid (crystal as in YAG and Ruby lasers or semiconductor as in diode laser), liquid (pulsed dye laser [PDL]), or gas (helium-neon laser)
• Determines the wavelength of the light emitted by the laser
■ Two parallel mirrors: One mirror 100% reflective, and the exiting mirror varies in reflectivity and how it releases energy
MECHANISM OF ACTION
■ Energy is transmitted from the excitation mechanism to the medium.
■ Electrons in the medium are excited to a higher-energy state.
■ As the electrons fall back to their baseline state, they release a photon.
■ Photons result in waves of light (energy) that can exit the medium to provide a uniform wavelength of light or that can hit an adjacent electron to further excite the medium and amplify the energy.
■ The light waves are reflected back and forth between the mirrors.
■ The exiting mirror can be manipulated so that only photons that hit it exactly perpendicular may exit, creating coherent waves that move in phase.
NOTE: Q-switching involves two mirrors that are 100% reflective. The exiting mirror shutters open, releasing the entire cavity at once. Q-switching creates very high powers.
PROPERTIES OF LASER LIGHT
■ In phase
■ Monochromatic
■ Coherent
EFFECT ON TARGET
■ Reflection
• Off shiny surgical instruments
• Off skin or target, which decreases energy delivered to target
• Can cause ocular damage if wavelength-specific goggles not worn
■ Scatter
• Off dull objects or targets that are just off the peak of absorbent coefficients
■ Transmission
• The less target in the medium, the more the light will be transmitted through the medium. This varies with each laser, each with its own specific targets.
■ Absorption
• Laser energy absorbed by target chromophore
• Energy converted to thermal energy in target chromophore (selective photothermolysis) or with extremely high energies and short-wavelength Q-switched lasers, which generate acoustic energy
• Usually is the mechanism of action that provides the desired effect
• Heat can transmit to surrounding tissues and cause a wider zone of damage if the principles of selective photothermolysis are not followed.
CHARACTERISTICS OF LASERS
■ Wavelength
• Each wavelength has absorption spectrum that identifies the optimal targets.
• Within the visible light spectrum (approximately 400-750 nm), the longer the wavelength, the deeper the penetration.
■ Pulse duration
• Continuous: More likely to cause nonselective tissue injury
• Long pulses (milliseconds)
• Short pulses (nanoseconds), as in Q-switched laser
■ Spot size
• Smaller spot sizes have more scatter and may therefore not reach the dermis.
• Generally, the larger the spot size, the deeper the penetration.
■ Beam shape
• Most lasers have a Gaussian distribution of intensity within the beam (lower intensity along the periphery).
• Therefore some overlap between treatment zones is needed.
■ Surface cooling: Allows protection of the epidermis while heating the deeper targets in the dermis
■ Pulse width: Duration of time tissues are exposed to the laser
■ Fluence: Energy delivered per surface area (J/cm2)
■ Thermal relaxation time: Time for tissues to lose 50% of their heat
MAJOR TYPES OF LASERS
ABLATIVE LASERS (see Chapter 17)
■ Chromophore is water.
■ Vaporize the epidermis and possibly part of the dermis
■ Cause mild to significant edema and an open wound, all dependent on the depth of injury
■ Have a higher risk of scarring and pigmentation changes than nonablative lasers
■ Erythema longer than nonablative and is depth dependent
■ Very effective for treating moderate to severe rhytids
■ Most commonly used ablative lasers: CO2 laser (10,600 nm) and the erbium:YAG laser (2940 nm)
NONABLATIVE LASERS
■ Spare the epidermis and cause thermal damage in the dermis
• Collagen is denatured when heated to 60°-70° C, stimulating new collagen formation.
■ During the healing response, fibroblasts are activated in the papillary and midreticular dermis, increasing type I collagen and elastin deposition ➤ dermal thickening.
■ Collagen reorganizes into parallel fibrils.2
■ Skin tightens, and irregularities decrease.3
TIP: Although nonablative lasers allow skin rejuvenation with minimal downtime, their efficacy is significantly less than that of ablative rejuvenation methods.
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